Abstract

Experiments have been performed to elucidate the mechanism of proton transfer in ternary clusters containing the matrix-assisted laser-desorption ionization (MALDI) matrix sinapinic acid, nonchromophoric analytes (proline, methionine, and prolylmethionine), and argon. To investigate the mechanism of intracluster proton transfer, ionizing laser power studies were performed at 266 and 355 nm. Baseline studies show that two photons are required at both wavelengths for the formation of sinapinic acid radical cations from sinapinic acid/argon clusters. Studies of the ternary sinapinic acid/biomolecule/argon clusters show that, in all cases, the photon dependence for protonation of the biomolecule is the same as that for formation of the sinapinic acid radical cation. Furthermore, the slopes of the power plots are generally between 1.5 and 2.0, consistent with a two photon ionization process. No evidence of negative ion formation is detected in the negative ion mass spectra. The combined results are consistent with a mechanism of biomolecular intracluster protonation via proton transfer from the photoionized sinapinic acid radical cation. Wavelength dependent trends in matrix and analyte fragment ion formation in conventional MALDI mass spectra and the cluster proton transfer mass spectra were noted. The possible contribution of cluster proton transfer to the analyte protonation mechanism in conventional MALDI is discussed.